The invention relates to a touch sensitive apparatus adapted to locate the coordinates of a point along a line disposed on a substrate, and more particularly to an apparatus for locating a point characterized by an impedance discontinuity in the line generated as a result of the line being contacted by a fingertip or a conductive segment.
Operator interaction with a computer system is greatly facilitated by use of input devices such as keyboards, touch panel devices, and the like. For some users, particularly those who are visually oriented, the art has responded with visually interactive means permitting the user to directly interface with the computer system via a visual display. Indeed there are now many touch control techniques from which the user can choose.
One type of touch control apparatus, commonly referred to as a capacitive touch device, employs a resistive surface having a bounded region which is used for coordinate detecting. In one such capacitive touch device, disclosed in U.S. Pat. No. 4,293,734 to Pepper Jr., the bounded region is a resistive surface having uniform resistivity, a rectangular shape and a terminal at each edge. In the two-axis form of the Pepper, Jr. device, input-output locations are provided for an x-axis and a y-axis. The ratio of the sum of the currents through two of the terminals to the sum of the currents through all four of the terminals is proportional to the distance from one edge. Accordingly, output voltages that are proportional to the x and y coordinates of the point touched are derived simultaneously.
As can be recognized by those skilled in the art, the Pepper Jr. arrangement uses a current ratio means of detecting location in much the same way as a bridge circuit would. In a similar manner, measurements are achieved with a capacitive touch device disclosed in U.S. Pat. No. 4,680,430 to Yoshikawa et al., in which current ratio means are employed to determine separate x and y locations.
Another capacitive touch device is disclosed by U.S. Pat. No. 4,476,463 to Ng et al. The Ng et al. system uses an electrically conductive touch sensitive coated surface with four elongated electrodes connected to the coating, one electrode being provided along each side of the touch sensitive coating. Measurements are made of the change in electrical impedance which a capacitive touch introduces to a resistance-capacitance circuit including the touch sensitive surface. Position of touch is determined by correlating measurements for x-y locations with corresponding capacitive ratio variations.
Still another touch panel device with a touch sensing surface is disclosed in U.S. Pat. No. 4,680,429 to Murdock et al. The Murdock et al. patent permits impedance sensing relative to multiple contacts. In the Murdock system, a touch location along the surface can be determined from the interaction of a finger with touch currents generated by selectively applying alternating current voltage panel scanning signals to the touch sensing surface.
There are distinct drawbacks to employing capacitive touch devices to detect coordinate locations on a resistive surface. In particular, a typical capacitive touch device must be designed with fairly wide spacing between signal traces in order to minimize the effects of stray capacitance. Accordingly, the number of touch sensitive areas per overlay is severely constrained which, in turn, limits the field of information on the display.
Additionally, determining coordinates in more than one dimension typically involves the processing of two or more signals using two or more dedicated circuits. For example, in a preferred embodiment of the Murdock et al. system, at least three currents must be detected in order to determine touch location. Measuring a number of currents and/or voltages as a prerequisite to determining coordinate locations not only promotes complexity in hardware requirements but is very prone to generating error.
Another type of touch control technique exploits the concept of wave propagation to achieve coordinate detection. For example, U.S. Pat. No. 4,689,448 to Snyder et al. is directed toward a two dimensional position coordinate determination device with a U-shaped delay line. A PC grid having conductors is used in conjunction with the U-shaped delay line, which line consists of first and second linear portions. When the grid is contacted by a pointer including a circular flux-producing element, strain waves are generated from the first and second linear portions. The strain waves are propagated along the U-shaped delay line toward a sensor coil. Propagation times associated with x and y coordinates are processed by logic circuitry to obtain the x and y coordinates.
Another wave propagating device, disclosed in U.S. Pat. No. 4,506,354 to Hansen, discloses an apparatus which uses a pair of ultrasonic transducers to determine the position of any of a variety of objects. In a preferred embodiment, the transducers are pulsed at several frequencies and a receiving means senses echoes from each pulse. Selected echoes are used to measure the distance of the object from the transducers to determine the position of the object.
Yet another wave propagating device, disclosed in U.S. Pat. No. 4,700,176 to Adler, employs surface acoustic wave propagation in conjunction with input-output transducers to detect a touch location. In a broad sense, the Adler arrangement may be thought of as an absorption ranging system. In a preferred embodiment, waves are transmitted across a touch surface from one boundary to another opposing boundary. When a fingertip is disposed on the touch surface, acoustic surface wave energy is absorbed and the amplitude of a surface wave burst propagating through the region of the touch is damped. Accordingly, the damping is sensed and the timing information generated thereby is employed to determine which of the plurality of burst propagation pads has been perturbed and thus the location of the touch.
While the wave propagating devices discussed above provide for a wide range of interaction with computer systems that cannot typically be achieved with capacitive touch devices, wave propagating devices still possess certain attributes that, in a significant number of situations, make their use particularly disadvantageous. For example, the arrangements of the Snyder et al., Hansen and Adler patents can be difficult to properly implement. The implementation for each of these patents requires precise positioning of components in order to achieve optimum results. Improper positioning of the transducers in the Adler and Hansen patents, or of the delay line in the Snyder et al. patent, can severely impair operation and generate erroneous results. Moreover, the construction required for each network is extensive, thus increasing manufacturing costs.
In view of the above discussion, a need exists for a touch sensitive apparatus that exploits the advantageous signal processing features of the wave propagating devices, but is no more difficult to implement than the capacitive touch devices. At the same time, the improved touch sensitive apparatus should include as few components as is absolutely necessary for acceptable operation, and yet provide a high degree of flexibility in design and operation.